An organic light-emitting diode (OLED) device is generally composed of glass substrates, conductive electrodes made of indium tin oxide (ITO), and an organic light-emitting layer. No matter whether the OLED device is a “top-emitting type” or “bottom-emitting type”, the large difference between the refractive indices of the materials used in the device results in reflection at the interface. This reflection in OLEDs may cause a low efficiency of light extraction. According to studies, in general OLEDs, almost a 70-80% loss of light that cannot be guided outside the device results from the reflection at the interface. The difference of the refractive indices of the materials used in OLEDs may be improved by selecting the materials of OLEDs or changing the structure thereof to increase light extraction efficiency. However, the alteration of materials or structures is accompanied by changes in the manufacturing process which brings a greater challenge to the development of OLEDs.
An external light extraction diffusion layer is typically disposed on the surface of OLEDs to achieve diffusion effects by light scattering. The diffusion layer is generally composed of solid organic particles or inorganic particles with a high refractive index. However, the effect of light extraction is unsatisfactory.
In previous studies, a diffusion layer comprising a microlens or solid inorganic particles have been used as a light-extraction film, or the diffusion layer can be composed of particles, ultrafine particles, and resin. Since the particles are surrounded by a high concentration of ultrafine particles with a large difference of refractive indexes, a good light-extraction efficiency can be obtained. However, the effect of reducing the color shift is limited. The color shift can be reduced by coating a diffusion layer (a particle with a high refractive index, the particle size being about 0.2-1 μm) and a light condenser layer (the refractive index of the particle being about 1.4-1.6, and the particle size being about 3-10 μm) sequentially on the transparent resin substrate. Although the combination of the two coatings as a light-extraction layer can reduce the color shift, the process is more complicated. In addition, a diffusion layer that includes scattering particles of a special shape can provide improved brightness and anti-adhesive, antifouling, and weather-proofing properties. However, such a diffusion layer needs materials of two different refractive indices.
Therefore, improvements in the light-extraction efficiency of the light-emitting element of OLEDs are required.